1. Field of the Invention
The present invention relates generally to an apparatus and method for communicating using spread spectrum techniques, and more particularly for an apparatus and method of locking onto a pseudo-noise code in an IS-95 spread spectrum communications system.
2. Discussion of Background Art
Spread-spectrum communication systems currently find widespread use in modern cellular communications devices. Spread spectrum systems allow more users to transmit and receive communications in an ever tighter bandwidth environment.
One technique for spreading a baseband signal so as to fill an entire channel bandwidth is to mix the baseband signal with a Walsh code and a complex pseudo-noise (PN) spreading signal. The Walsh code and PN spreading signal effectively encode the baseband signal by modulating (i.e. chopping) each data symbol within the baseband signal into a number of chips having a chip period (i.e. chip interval) TC, as is discussed further by Charles E. Cook and Howard S. Marsh, xe2x80x9cAn Introduction to Spread Spectrum,xe2x80x9d IEEE Communications Magazine, March 1983, and by David P. Whipple, xe2x80x9cThe CDMA Standardxe2x80x9d, Applied Microwave and Wireless, Winter 1994, pp. 24-39 (originally published as, xe2x80x9cNorth American Cellular CDMAxe2x80x9d, Hewlett-Packard Journal, December 1993, pp. 90-97). The complex PN code is given by the following equation: PN(t)=PNI(txe2x88x92xcex4)+jPNJ(txe2x88x92xcex4), where xcex4 is a phase offset. Each transmitter within a CDMA network broadcasting over the same frequency spectrum and within a distance of a particular receiver is distinguishable by its unique phase offset, xcex4. Each of the transmitters include a number of channels which are encoded and distinguished by different Walsh codes.
Current spread spectrum receivers acquire many different transmitted signals, which, while appearing to be superimposed on one another, are demodulated by correlators that are tuned to accept only transmitted channels corresponding to a particular PN code phase offset and a particular Walsh code. The receiver accomplishes this by stripping away the carrier signal and demodulating the spread spectrum signals with correlators having a matching PN code phase offset and Walsh code.
In order for demodulation to occur successfully, the transmitter""s and receiver""s PN spreading phase offsets must be synchronized. Delay-locked Loops (DLLs) containing correlators are commonly used to synchronize the receiver""s PN code phase offset to the transmitter""s PN code phase offset. However, the filters within DLLs reduce the orthogonality of the Walsh codes used to encode the baseband data signal. As a result, the baseband data signals are more susceptible to being corrupted by Orthogonal Channel Noise (OCN) and Multiple Access Interference (MAI) noise. High levels of MAI noise also significantly increase a probability that the DLL will lose lock, disrupting communications.
What is needed is an apparatus and method for reducing the effect of noise on delay locked loops within spread spectrum communications systems conforming to the IS-95 standard.
The present invention is a spread spectrum communication receiver that is compliant with IS-95 standards and incorporates a modified delay locked loop which is more noise tolerant than prior art devices. In response to an error signal, a pseudo-noise generator within the circuit of the present invention generates, an on-time/punctual pseudo-noise signal, an early pseudo-noise signal delayed by less than one-half of a chip interval, and a late pseudo-noise signal advanced by less than one-half of a chip interval. A correlator mixes and integrates a received signal with the early and late pseudo-noise signals to generate the error signal. The combination of the pseudo-noise generator and the correlator create a loop in which the on-time pseudo-noise signal tracks a received pseudo-noise code more closely and enables a deinterleave and decode device within the receiver to demodulate the received signal more effectively.
The method of the present invention includes the steps of generating an on-time pseudo-noise signal, an early pseudo-noise signal delayed by less than one-half of a chip interval, and a late pseudo-noise signal advanced by less than one-half of a chip interval, in response to an error signal, and then correlating a received signal with the early and late pseudo-noise signals to generate the error signal, thereby completing a delay-locked loop circuit for tracking a received pseudo-noise code.
These and other aspects of the invention will be recognized by those skilled in the art upon review of the detailed description, drawings, and claims set forth below.